1. Field of the Invention
This invention relates to a method for manufacturing semiconductor modules such as semiconductor laser modules and semiconductor amplifier modules used in optical communication. More particularly, this invention relates to a method for manufacturing a semiconductor module, in which a high-coercivity film (a film with high coercive force) is not removed from its magnetically saturated condition when a semiconductor module is manufactured and which is incorporated with an optical isolator making use of a high-coercivity film magnetic garnet film in which a magnetically saturated condition is maintained even without any external magnetic field.
2. Description of the Related Art
In the semiconductor laser modules and semiconductor amplifier modules used in optical communication, optical isolators are incorporated in order to stop light from returning to semiconductor elements such as semiconductor lasers. An example thereof is described with reference to FIG. 1.
First, a semiconductor laser module of this type is chiefly constituted of, as shown in FIG. 1, a housing 11, a photodiode 4 disposed in this housing 11 along an optical axis P, a semiconductor laser element 1, a focusing lens 5, an optical isolator 7, and also an optical fiber 8 guided into the housing 11 through a ferrule 9 fitted to a sidewall of the housing 11.
Then, this semiconductor laser module is manufactured through the steps of an assembly described below.
First, the semiconductor laser element 1 is mounted on a semiconductor laser carrier 3 via a heat sink 2. This semiconductor laser carrier 3 is soldered to a base 6 to which the focusing lens 5 has been fastened together with the photodiode 4. On this base 6, the optical isolator 7 is also mounted, and the optical fiber 8 guided into the housing 11 through the ferrule 9 is optically adjusted to the optical isolator 7. Thereafter, these are each fastened.
The base 6 is also fastened to the housing 11, which is made of a metal, through a Peltier element 10, and also the ferrule 9, which protects the optical fiber, is soldered to the housing 11 at its optical-fiber guide-in hole 15.
The semiconductor laser module is further electrically connected to a lead (not shown) in the housing 11 by wiring making use of a bonding wire; the lead projecting to the outside of the housing 11. Thereafter, the housing 11 is closed by fastening thereto a metallic cover 12 at its opening by soldering, and the interior of the housing 11 is made air-tight by hermetic sealing, thus the semiconductor laser module is completed.
Incidentally the optical isolator 7 is constituted of a Faraday rotator formed of a magnetic garnet film and a pair of polarizers disposed on both sides, and a small-sized and strong permanent magnet for saturating the Faraday rotator magnetically is disposed around the Faraday rotator.
Now, in recent year, there is an increasing demand for making the semiconductor laser module compact. With this demand, the space inside the metallic housing has come narrower.
In order to meet the demand for making it compact, a Faraday rotator has been developed which is formed of a special magnetic garnet film that requires no magnet (see Japanese Patent Application Laid-open No. H9-185027).
This magnetic garnet film is called a high-coercivity film because, after it has initially magnetically been saturated by applying an external magnetic field, the magnetically saturated condition is maintained even when the external magnetic field is removed.
Where an optical isolator is made using the Faraday rotator formed of this high-coercivity film, the space inside the metallic housing can be made narrower insofar as it requires no magnet, and such an optical isolator has an advantage in respect of cost.
Now, in the manufacture of semiconductor modules such as the semiconductor laser module, there are various heating steps for the above soldering after the optical isolator has been incorporated in the housing. In some cases, the fastening itself of the optical isolator is carried out by soldering. Also, even where the optical isolator is fastened with an adhesive in place of soldering, heating to about 120° C. is necessary in order to solidify the adhesive, and hence there still is the step of heating.
Even in such a semiconductor module manufacturing process having the steps of heating, there has been no particular problem in the case when the above conventional optical isolator making use of a permanent magnet is incorporated.
However, in the case when the optical isolator making use of a high-coercivity film is incorporated, there has been a problem in that the high-coercivity film that has been fastened with much trouble is undesirably removed from its magnetically saturated condition as a result of the heat treatment for soldering, to return to the original condition of a multiple magnetic domain.
Also in the case when the method is employed in which the optical isolator is fastened with an adhesive in place of soldering, the high-coercivity film may be removed from its magnetically saturated condition as a result of the heat treatment when the adhesive is made to solidify. Once the high-coercivity film has been removed from its magnetically saturated condition, the optical isolator no longer functions as the optical isolator, and hence, where the optical isolator making use of a high-coercivity film is incorporated in the semiconductor module, it has been necessary for the adhesive to be solidified at a relatively low temperature, or to again saturate the high-coercivity film magnetically (hereinafter called “re-magnetize”) after the semiconductor module has been assembled.
However, the fastening with an adhesive has had a problem that it involves a lower reliability than the soldering. Also, in order to re-magnetize the high-coercivity film after the semiconductor module has been assembled, it is necessary to heat the semiconductor module to about 100° C. and apply a magnetic field of as strong as 1.6×105 A/m or more from the outside. Hence, this requires an elaborate magnetic-field generation system, and also has caused a problem that the high-coercivity film can not be re-magnetized because no magnetic field is applicable thereto if a magnetic material is present in the semiconductor module.
The present invention has been made taking note of such problems. An object of the present invention is to provide a semiconductor module manufacturing process in which the high-coercivity film is not removed from its magnetically saturated condition even when the step of heating at 100° C. or more is present in the course of manufacture when the semiconductor module incorporated with the optical isolator making use of a high-coercivity film is manufactured.
Accordingly, the present inventors have made extensive studies in order to achieve such an object. As a result, they have reached a technical finding as stated below.
That is, they have discovered that, although the high-coercivity film is removed from its magnetically saturated condition upon an increase in temperature to come into the condition of a multiple magnetic domain so as to not function as the Faraday rotator, it is not removed from its magnetically saturated condition as long as an external magnetic field is present in the same direction as the direction of magnetization of the high-coercivity film even when the magnetic field is weak, namely, where the film is exposed to a temperature at which the Faraday rotator comes to have a small coercive force, its magnetization is retained as long as it is cooled in a state in which an external magnetic field not lower than the coercive force after having been reduced is applied.
The present invention has been accomplished based on such a technical finding.